Disposable products made from papermaking fibers often serve as substitutes in both the home and in industrial shops for conventional cloth wipers and towels. Such paper products must closely simulate cloth in both consumer perception and performance. Such wiper products, including paper towels, industrial wipers, and other similar products, are designed to have several cloth-like properties.
For example, paper wiper products should exhibit good bulk, have a soft feel, have adequate strength even when wet, have good stretch characteristics, and resist tearing. These products should be highly absorbent and be abrasion resistant, and should not deteriorate in the environment in which they are used.
In the past, many attempts have been made to enhance certain physical properties of disposable wiping products. Unfortunately, however, when steps are taken to increase one property of a wiping product, other characteristics of the product may be adversely affected. For instance, in cellulosic-based wiping products, softness is typically increased by reducing cellulosic fiber bonding within the paper product. Inhibiting fiber bonding, however, usually adversely affects the strength of the paper web.
One method that has been employed to reduce the stiff papermaking bonds is to crepe the paper from a drying surface with a doctor blade, which disrupts and breaks many of the interfiber bonds in the paper web. Other methods reduce these bonds by preventing formation of the bonds, rather than breaking them after they are formed. Examples of these other methods are chemical treatment of the papermaking fibers to reduce their interfiber bonding capacity before they are deposited on the web-forming surface, use of unrefined fibers in the slurry, inclusion into the slurry of synthetic fibers which are unable to form papermaking bonds, and use of little or no pressing of the web to remove the water from the paper web after it is deposited on the web-forming surface. This latter method reduces formation of bonds by reducing close contact of the fibers with each other during the forming process. Although these methods successfully increase the softness of paper webs, they result in a loss of strength in the web.
Attempts to restore the strength lost by reduction of papermaking bonds have included the addition to the web of bonding materials that add more strength than stiffness to the web. Such bonding materials may be added to the aqueous slurry of fibers and deposited on the web-forming surface along with the fibers. With this method, the bonding material can be distributed evenly throughout the web, avoiding the harshness which would accompany concentrations of bonding material. However, this method has the disadvantage of reducing the absorbency of the web by filling the pores between the fibers with bonding material.
Another method which has been used to apply bonding material to the web is to apply the bonding material in a spaced-apart pattern to the web. In products made by this method, the majority of the web surface does not contain absorbency-reducing bonding material. This method is commonly employed in the field of nonwovens where little or no strength is imparted to the web by papermaking bonds, and almost all of the strength is obtained from the bonding materials.
In contrast to nonwoven webs, webs made entirely or principally from papermaking fibers require bonding areas to be quite close together because papermaking fibers are very short, generally less than one-quarter of an inch long. Thus, it has been thought that to apply sufficient bonding material in a pattern to a paper web to the degree necessary to bond each fiber into the network would result in a harsh sheet, having poor softness characteristics, particularly in the areas where the bonding material is located.
Another method that reduces the harshness in the web area where the bonding material is concentrated consists of forming a fibrous web under conditions which result in very low interfiber bonding strength by one of the previously described methods. Strength is then imparted to the web by apply bonding material to one surface of the web in a fine spaced-apart pattern. The harshness in the bonded areas is reduced by tightly adhering bonded portions of the web to a creping surface and removing the single-side bonded web with a doctor blade, thus finely creping the bonded portions to soften them. This form of controlled pattern creping also results in a number of other property improvements. For example, selective creping of the bonded areas in the surface of the web creates contraction of the web surface in all directions, resulting in an increase in stretch in both the machine direction and the cross-machine direction of the web. Also, the portions of the web where the bonding material is not located are generally disrupted by the creping action, resulting in an increase in bulk of the web, an increase in the softness of the web, and an increase in absorbency. At certain locations within the web, generally close to the bonding material locations, the web may develop internal split portions which further enhance the absorbency, softness, and bulk of the web. This effect does not occur, at least to the same extent, in the web formed by addition of bonding material to the aqueous slurry of fibers.
This one-sided bonded/creped method produces a paper web with high softness and strength, two properties which were previously believed to be almost mutually exclusive in paper webs. It also produces a web with adequate absorbency properties due to the bonding material being confined to only a portion of the web surface. Furthermore, the compaction of the surface fibers due to the shrinkage of the areas containing bonding material creates one surface of the web which has improved wipe-dry characteristics upon being finely creped. It is also believed that pressing the web to the creping surface while the web has moist portions on the surface region due to the uncured or undried bonding material causes the fibers in those moist areas to compact.
This method is particularly useful in production of webs in a lower basis weight range for use such as bathroom tissues. However, it has shortcomings in making webs for heavier duty use such as for towels and wipers where greater strength, bulk and absorbency is desired. Examples of such shortcomings are poor abrasion resistance on the nonbonded side of the web and less strength than may be desired. Both of these properties could be improved by causing the bonding material to penetrate completely through the web to create a network of bonding material on both sides of and entirely through the web, but it has been found that such one-side bonded, thoroughly penetrated webs would have less of the improvements described above.
For example, bonding the web with the bonding material extending completely through the web would greatly reduce the disruption of the fibers within the web upon creping and, therefore, result in a reduction of bulk, softness, and absorbency. Also, complete penetration of the bonding material through the web is difficult to accomplish on heavier basis weight webs and attempts to do so result in concentrations of excess bonding material at the web surface where much of it is ineffective for strengthening interfiber bonds. Furthermore, if complete penetration of the bonding material does result, the bonding material in the interior of the web will not be as efficiently used to increase abrasion-resistance of the web as when it is placed only in the surface of the web. Placement of the bonding material in the interior of the web is not only an inefficient use of the expensive bonding material, but results in a harsher feel to the web due to the inability of the creping action to soften the bonded portions as effectively. Also, bonding completely through the web would reduce the ability to create on both sides of the web a web surface of compacted fibers having good wipe-dry characteristics while at the same time creating a bulky web capable of absorbing a larger amount of moisture. These properties are only of minor importance when producing a product for such uses as bathroom tissues, but where the product is to be used for wipers or towels, they are very important.
One particular process that has proved to be successful in producing paper towels and other wiping products is disclosed in U.S. Pat. No. 3,879,257 to Gentile, et al., which is incorporated herein by reference in its entirety. In Gentile, et al., a process is disclosed for producing soft, absorbent, single-ply fibrous webs having a laminate-like structure that are particularly well suited for use as wiping products.
The fibrous webs disclosed in Gentile, et al. are made from a fibrous web formed from an aqueous slurry of principally lignocellulosic fibers under conditions which reduce interfiber bonding. After formation, the web is usually creped prior to further processing. A bonding material, such as a latex elastomeric composition, is then applied to a first surface of the web in a spaced-apart pattern. In particular, the bonding material is applied so that it covers from about 50 percent to about 60 percent of the surface area of the web. The bonding material provides strength to the web and abrasion resistance to the surface. Once applied, the bonding material can penetrate the web preferably from about 10 percent to about 40 percent of the thickness of the web.
The bonding material is then similarly applied to the opposite side of the web for further providing additional strength and abrasion resistance. Once the bonding material is applied to the second side of the web, one side of the web is brought into contact with a creping surface. The web adheres to the creping surface according to the pattern to which the bonding material was applied. The web is then creped from the creping surface with a doctor blade, which greatly disrupts the fibers within the web where the bonding material is not disposed, thereby increasing the softness, absorbency, and the bulk of the web.
In a preferred embodiment disclosed in Gentile, et al., each side of the paper web is creped after the bonding material has been applied to the side. Gentile et al. also discusses the use of chemical debonders to treat the fibers prior to forming the web in order to further reduce interfiber bonding and to increase softness and bulk.
Another method employed to produce a wiper-like paper product having the desirable bulk, absorbency, and abrasion-resistance, is to laminate two or more embossed conventional paper webs together with an adhesive. One advantage of this method is that the tightly compacted fibers of the conventional paper webs offer good wipe-dry properties on both sides of the sheet while, at the same time, the void spaces between the webs created by the embossments spacing the webs from each other increase the ability of the web to hold moisture. Examples of this method are disclosed in U.S. Pat. Nos. 3,414,459 and 3,556,907. The disadvantages of this method are apparent when considering the complex process involved in separately embossing two or more webs and then bringing them together with synchronism to prevent complete nesting of the embossed protuberances of one web into the embossed protuberances of the other web. Also, any given length of the multi-ply product requires initial production on a papermaking machine of a web two or more times as long. It is also apparent that the adhesive used to interconnect the plies to each other will present unpleasant stiffness at the location where the adhesive is disposed.
Multi-ply embossed paper products, however, are quite desirable in that they can be made very bulky compared to their weight, due to the void spaces between the plies created by the embossed protuberances holding the plies apart from each other. Because of this construction, multi-ply products are easily compressed between the finger of the consumer, thereby aiding in providing a feeling of softness.
The processes disclosed in Gentile et al. have provided great advancements in the art of making disposable wiping products. The products, however, tend to be somewhat expensive, in part, because two printing (or latex bonding) processes and two creping processes are generally involved. Thus, it would be desirable if disposable wiping products having properties similar to those disclosed in Gentile et al. could be produced at lower costs.